1. Field of the Invention
This invention relates to the techniques of producing curved and possibly also thermally toughened glass sheets, notably those curved to complex, non-cylindrical shapes. More particularly, the invention concerns such techniques in which the glass sheets are caused to pass over a shaping bed composed of shaping rods, for example revolving elements, disposed along a path having a profile curved in the direction of travel of the glass sheets. The invention is applicable, for example, to the production of automobile glazing panes, for example of the fixed side window type.
2. Description of the Related Art
These curving-toughening techniques are known, for example, from French Patent 2 442 219, which proposes to preheat the glass sheets to the softening temperature, and then to cause them to travel on a conveyor continuing uninterruptedly the path followed by the glass sheet from the furnace, said conveyor forming a shaping bed passing through a final toughening zone and being constituted of revolving elements disposed along a path having a curved profile, in practice a circular profile with its concavity preferably facing upwards. In this case, the trajectory followed by the glass sheet is not plane but cylindrical, the generatrices of the cylinder being horizontal and preferably perpendicular to the direction of feed of the glass, the glass being brought flat into the installation. The radius of the cylinder to which the trajectory of the glass sheet fits corresponds to the radius of curvature imparted to the glass sheet in the direction parallel to its direction of travel.
With revolving elements composed of straight rods, a right cylinder is obtained. Other toroidal forms of revolution are obtained by substituting for the straight rods shaped rollers, for example of the handlebar type, bulging rollers associated with diabolos, or rods with contraflexure. Depending upon the curvature of the revolving elements, it is thus possible to achieve toroidal pane shapes, the radius of curvature of which varies from 1 meter to infinity for the principal curvature and from 20 meters to infinity for the secondary curvature, infinity corresponding to straight rollers and therefore to a cylindrical shape.
Although shaped rollers make possible a wide variety of pane shapes, they nevertheless have many disadvantages. Firstly, it is in practice very difficult to use them in the absence of upper counter-rollers, contrary to the case of straight rods (see EP-A-263 030). Also, now each upper counter-roller must have a shape rigorously complementary to that of the corresponding lower roller and the control of the shape of a roller is quite evidently more complicated to achieve if this shape departs from a simple straight rod mounted without contraflexure. Furthermore, they are more expensive and this cost is increased by the need for maintaining stocks. Finally, considerations of an essentially technical nature limit the minimum secondary radius of curvature to 20 meters, which is not a very pronounced curve.
The lateral panes of automobile vehicles today constitute the primary target for these curving techniques, which advantageously are carried out at very high production rates due notably to the possibility of allowing the glass sheets to follow one another at spacings of only a few centimeters between them. Moreover, these curving techniques make possible very high reproducibility of the curvature and of the optical quality of the final panes, which is especially appreciated from a industrial aspect. Nevertheless, these curving techniques cannot be used in all cases where the intended final shape has at least one fold line, following the example of those panes known as flush, of which the principal surface is tangential to the remainder of the automobile body when they have been installed in their frames. In fact, these panes of more complex shapes cannot at present be produced except by application against a curving mold and very frequently a complementary pressing against a curving frame, which of necessity requires an interruption to the passage of the glass sheets and a relatively wide spacing between them (while the first sheet is in process of being curved, the curving tool or mold is unavailable throughout the duration of curving, whereas in an installation such as that described in French Patent 2 242 219, the outer edge of a glass sheet can penetrate into the shaping zone while the rear edge of the preceding sheet is still there).
Apart from this aspect which limits the rates of production, the problem must also be faced of perfect mastery of the curvature effectively imparted during curving and of the prevention of any parasitic deformation due, for instance, to sagging of the glass under its own weight in those zones where it is not supported. These parasitic deformations generally take place during the transfer of the glass sheet from the curving station to the toughening station and they become all the more difficult to avoid as the curvature imparted to the glass sheet becomes less, which means that the glass sheet still has a great tendency to curve which is difficult to master.
This problem of parasitic curvatures is especially harmful in the case of the lateral panes equipped with raising and lowering devices, which require that the glass sheets shall have no or virtually no parasitic curvature, so that they may slide easily in the seating of the door provided for this purpose. In contrast, the curving-toughening methods on a shaping bed having a curved profile provide a very satisfactory solution to this problem of mastering the curvature, on account of the permanent support for the glass sheets provided by the rollers and the fact that the glass sheets are toughened directly after they are curved, without having to be transferred from a curving station to a toughening or quenching station.
Quite apart from this aspect of properly controlling the curvature, the techniques having a pressing frame permit much lower production rates than those achieved with the curving-toughening processes in which the glass sheets travel continuously without stopping. Finally, even though such panes can be considered as relatively complex by reason of their fold very near to an edge of the sheet, they are still nevertheless basically of cylindrical shape and therefore a curving production line intended for the production of the most complex panes would be over-dimensioned and therefore ill adapted.
It should, moreover, be noted that is it known from Patent Application FR-A-2 221 409 to curve glass in the form of strips or sheets by bringing it onto a plurality of conveying parallel rollers divided into two layers of parallel rollers, the planes containing the axes of the rollers of each layer being at an angle to each other and their intersection being perpendicular to the axes of the rollers, the apex of the angle formed by these two planes being orientated downwards. In the case of an individual glass sheet, cut to shape in advance, the sheet is brought into the curving station, the rollers of which are kept horizontal, and then when the sheet is in position the desired inclination is simultaneously given to perform the shaping.
Such a process is intended for the production of shapes having a developable surface, the rollers of each layer being disposed in a single plane. In the case of the volumes or bodies to be treated by the present invention, a principal curvature is added or, more exactly, precedes, this principal curvature being most commonly cylindrical, with the result that the shape aimed at is not developable (and this is achievable only because of the capacity of the glass to flow). Furthermore, this principal curvature greatly stiffens the glass body, which means that it becomes very difficult to fold it transversely to the line of principal curvature.
Moreover, in the process described in the Patent Application FR-A-2 221 409, the curvature depends very much upon the temperature acquired by the glass. It is well known that this temperature may--for various reasons--vary in the course of time by several degrees, which leads to a certain inaccuracy in the shapes obtained, an inaccuracy which must be compensated by passage between a pair of calibrating curved rollers which pinch the glass sheet. During this complementary curving between these calibrating rollers, the glass continues to cool with the result that it becomes difficult to toughen it thermally, unless it is overheated in the furnace, which has a harmful effect upon the optical quality.